This invention relates generally to electronic circuitry, and more particularly, but not by way of limitation, to encapsulants for maintaining the electrical and mechanical integrity of solder connections between electronic devices and substrates.
Integrated circuit chips (ICs) for controlling electronic devices are currently cut from silicon wafers and packaged so that they can be electrically attached to circuitry of a substrate, such as a printed circuit board (PCB) or printed circuit cable (PCC).
IC packages have been known to take many forms. One example of a well-known IC package is the xe2x80x9cflip chipxe2x80x9d, in which an IC, or xe2x80x9cdiexe2x80x9d, has numerous electrical leads which are typically connected to solder balls, also known as solder xe2x80x9cbumpsxe2x80x9d on a surface of the die. The die is then inverted so the solder balls face the substrate, which is provided with contact pads on its upper surface. The solder balls on the die and the pads on the substrate are arranged so as to align with one another when the die is properly positioned on the substrate. After the die is placed on the substrate such that the solder bumps are in contact with the pads of the substrate, the assembly is heated so the solder is caused to reflow (i.e., liquefy). Upon cooling, the reflowed solder hardens, thereby forming structural solder joints. These joints electrically connect the die and substrate, and also provide overall structural support for the die-substrate assembly. A narrow gap is left between the die and the substrate.
It is desirable to provide reinforcement to the solder joints so they do not break under conditions such as vibration or thermal shock. One method for enhancing the reliability of the die-substrate assembly is to dispense an encapsulant in the gap between the die and the substrate. Encapsulants are known to take different forms including those of silicone, epoxy, or other organic resin systems. Of these families of materials, curable resins, i.e., resins that require a curing process after dispensing, are desirable because of their distinct physical properties before and after the curing or crosslinking process. Prior to curing, they are fluid so they can easily be dispensed into the chip-substrate gap so as to cover all exposed surfaces. After curing, they exhibit toughness, adhesion, and solvent resistance. The encapsulant is therefore able to provide enhanced reliability to the assembly by distributing mechanical stresses away from the solder bumps and also by encapsulating the solder bump interconnection so that they are not subject to environmental degradation. To a resin system determines the cured properties of the material, Generally, cured epoxy resins provide hardness and adhesion strength in most applications, but may exhibit a tendency to crack during thermal stressing. Silicones, on the other hand, are Less brittle than epoxies and are therefore less prone to fracturing under physical stress, but are also less rigid.
Traditionally, encapsulants have been introduced into the die-substrate gap by capillary flow underfill, a process which takes place after the solder joints have been wetted, heated to achieve reflow, and cooled. Underfill is accomplished by dispensing the resin about the perimeter of the gap and allowing capillary action to draw the resin into it. The assembly is then heated to a temperature sufficient to crosslink the resin to form the cured encapsulant.
While capillary flow underfill is an effective method by which to structurally reinforce solder joints, it requires a large number of manufacturing steps. These steps include: (1) fluxing the solder bumps on the die; (2) placing the die on the substrate; (3) heating to reflow the solder; (4) allowing the solder joints to cool; (5) reheating the substrate to the desired temperature to achieve proper flow of the underfill under the die; (6) introducing the encapsulant between the die and substrate; (7) heating the assembly again to cure the encapsulant; and finally, (8) allowing the assembly to cool once again. Performing all of these steps is both time-consuming and costly.
Reflow encapsulants have been introduced in recent years in an attempt to reduce the number of manufacturing steps associated with underfill encapsulation. Reflow encapsulants are similar to underfill encapsulants in that they are composed of a curable components, but differ in that one or more of the formulation components acts as a fluxing agent for the solder balls. Wetting of the solder bumps can then be accomplished by introducing the reflow encapsulant to the solder pads, eliminating the need for a separate fluxing step. In addition, reflow and curing can be accomplished together in one heating step, thereby eliminating the need for two separate heating steps. Assembly using a reflow encapsulant requires only the following steps: (1) applying the encapsulant to substrate or the chip; (2) placing the chip on the substrate; and (3) heating the assembly so as to both reflow the solder and cure the encapsulant. Use of a reflow encapsulant is clearly more efficient than underfill encapsulation in terms of both time and cost as a result of the elimination of one of the heating steps and the wetting step.
However, reflow encapsulants raise another set of problems. An epoxy formulation used for encapsulation typically has a coefficient of thermal expansion (CTE) significantly higher than that of a typical substrate material. Device failure can result when changes in temperature cause the substrate and encapsulant to expand at different rates. For this reason, it has been common to lower the CTE of the encapsulant by adding inorganic filler material to the epoxy formulation, typically in an amount of 50% to 70% by weight. This is satisfactory in the case of underfill encapsulation, where solder joints have already been formed upon introduction of the encapsulant. However, reflow encapsulant is introduced before solder joint reflow, and this raises the possibility that filler material in the encapsulant will prevent or hinder proper solder wetting during reflow. Improper solder wetting or flow can electrically and mechanically impair the integrity of the solder joints connecting the die and substrate.
What the prior art has been lacking is a reflow encapsulant which maintains the integrity of solder joints during reflow but which also does not lead to device failure as a result of temperature change.
Disclosed is a reflow encapsulant for use with a substrate and an electronic device. The encapsulant is configured to cure when the assembly is heated so as to reflow solder bumps connecting the substrate and electronic device. The encapsulant includes inorganic filler in an amount of 8% to 20% by weight. The amount of filler provided is sufficiently high to lower the CTE of the encapsulant so as to enhance cured material properties and prevent undue expansion and solder joint damage, but low enough so that the solder joints are not affected by filler particles during reflow.
These and various other features and advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.